Polyimides generally have superior heat resistance, mechanical properties and electrical characteristics compared to other general-purpose resins and engineering plastics. And also, wide applications of polyimides are founded such as molding materials, composite materials, electrical and electronics materials, optical materials, etc. Meanwhile, the use of lead-free solder in the manufacture of electrical circuits, including flexible printed boards, has become mainstream due to the increasing trend in environmental issues. Due to the high reflow temperature required for lead-free solder, polyimides that offer higher heat resistance than conventional ones are in increasing demand.
One effective technique for increasing the glass transition temperature (Tg) of polyimide resin is to optimize the monomer skeleton that constitutes the polyimide. For example, a diamine compound or tetracarboxylic dianhydride having a particular structure is introduced or copolymerized with conventional polyimide structure in an aim to improve the physical properties, such as heat resistance and/or mechanical properties, of the polyimide (see, e.g., Patent Literatures 1 and 2). However, new monomers are sometimes not versatile materials because of possible adverse effects on other physical properties, the difficulty with which they are synthesized, or the use of expensive raw materials.
Another effective technique for increasing the Tg of the resin is to introduce a functional group that undergoes thermal crosslinking into a terminal of the polyimide (see Patent Literature 3). However, this method involves altering the molar ratio between the added monomers. This inevitably results in a decrease in the molecular weight of the polyimide resin, possibly affecting its physical properties.
Under the foregoing circumstances, there has been a growing demand for technology that can increase the heat resistance of polyimide to a level higher than that of conventional one without changing its primary structure.
Bis(aminomethyl)cyclohexane is a diamine having an alicyclic structure. Thus, a polyimide prepared by reaction of this compound with an aromatic tetracarboxylic dianhydride is a semi-aromatic polyimide, a compound that exhibits high transparency (see Patent Literature 4). For this reason, bis(aminomethyl)cyclohexane holds great promise for future applications.
Bis(aminomethyl)cyclohexane has two structural isomers: 1,3-bis(aminomethyl)cyclohexane and 1,4-bis(aminomethyl)cyclohexane, each of which is known to exist as two geometric isomers: cis-trans isomers.